There are today many mobile communication systems that use synchronised time division schemes, such as time division duplexing systems (TDD). One example of this is Time Division-Long Term Evolution (TD-LTE). These types of systems are provided with base stations, which have certain communication structures, such as frames or superframes provided after one another in time. The communication structures of the base stations are in these types of systems synchronised with each other. The base stations of many systems do more particularly often transmit signals in one period of the structure, often denoted downlink period, and receive signals in another period of the structure, often denoted uplink period. These periods should be sufficiently separated from each other in order to avoid that base station transmissions interfere with base station reception. Such a separation is in some systems denoted a guard period. If systems are synchronised this means that all base stations are transmitting at the same time and all are receiving at the same time.
There exist some prior art documents that adjust these communication structures.
WO 2008/103090 does for instance disclose measuring the interference level during part of the time between two consecutive down link periods and varying the duration of guard periods according to the interference level.
WO 2009/153622 describes determining, through an apparatus equipped with a transmitter that monitors signal energy on a shared radio resource, the presence of other transmitters, identification of a predetermined set of time division duplex (TDD) signaling patterns, performing of a correlation between signal energy received on the shared radio resource and the predetermined TDD signaling patterns and determining of a TDD signaling pattern that may reduce or avoid interference with other transmitters using the shared resource.
One problem that has received increased attention lately in relation to systems using synchronised time division schemes are problems caused by atmospheric ducts.
An atmospheric duct is a horizontal layer that is created in the lower atmosphere, typically the troposphere. In such a duct the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted along the length of the duct. The radio signals in the ducts therefore tend to follow the curvature of the Earth. They also experience less attenuation in the ducts than they would if the ducts were not present.
The occurrences and locations of these ducts are hard to predict. They occur rarely, typically a few days or a number of hours in one year, and also often occur in some special area, like in the desert or near the sea.
For a mobile communication system, an atmospheric duct will cause long-distance downlink (DL) signals for base stations to mobile stations to travel through the atmosphere with long transmission delay but with a very low attenuation.
This delayed but still strong signal may seriously affect TDD system performance.
Since the base stations in the above mentioned systems are supposed to transmit in well controlled periods of the communication structure and to receive signals from mobile stations in other periods of the communication structure, it is possible that the signals from a base station travelling through an atmospheric duct will reach another base station when this other base station is supposed to receive signals from mobile stations, i.e. when this other base station is in uplink communication.
This situation can be serious because base stations transmit with much more power than a mobile station. Since the signals in the atmospheric duct have low attenuations, this means that a base station may not be able to receive any signals from mobile stations in the whole or parts of the period of the communication structure assigned to reception of signals from mobile stations. This means that one of the fundamental base station functions may be impossible to perform.
It is also possible for a base station to interfere with uplink communication of another base station for other reasons, like if it has a faulty operation, like having lost its synchronisation.
There is therefore a need for a solution to this problem.